The present invention relates to a process for the preparation of toluenediamine (TDA), in which dinitrotoluene (DNT) is reacted with hydrogen in the presence of a catalyst, wherein the dinitrotoluene used has a content of carbon dioxide in physically dissolved or chemically bonded form of not more than 0.175 mol %, based on the molar amount of the dinitrotoluene used.
Toluenediamines are intermediates for the preparation of toluene diisocyanates (TDI), which are important preliminary products, produced on a large scale, for the preparation of polyurethanes. Their preparation by catalytic hydrogenation of dinitrotoluenes (DNT) is known and has often been described (see, for example, Ullmann's Enzyklopädie der technischen Chemie, 4th Edition, Volume 7, page 393 ff, 1973, Verlag Chemie Weinheim/New York). The industrial production of toluenediamines is carried out predominantly by reaction of a mixture of isomeric dinitrotoluenes that is obtainable by nitration of toluene with nitric acid. Commercial mixtures of isomeric dinitrotoluenes are produced predominantly in the form of crude DNT in a two-stage isothermal nitration process using nitric acid in the presence of sulfuric acid as catalyst, with the formation of the corresponding mononitrotoluenes as intermediates. They are subsequently worked up in stages provided downstream of the reaction, predominantly in washing stages, and thus largely freed of dissolved sulfuric acid and nitric acid, and also of secondary components such as, for example, cresols and their degradation products, formed in the reaction stages.
Typical commercial DNT products have DNT contents >98.5% by weight, less than 0.1% by weight of mononitrotoluene, less than 0.1% by weight of trinitrotoluene and less than 0.1% by weight of other secondary components, as well as small residual amounts of toluene, based on the total weight of the DNT product mixture, with DNT yields of >98% and toluene conversions of >99.9%. Also important is the weight ratio of the total amount of the 2,4- and 2,6-DNT isomers to the total amount of the 2,3-, 3,4-, 2,5- and 3,5-DNT isomers. According to commercial specifications, the total content of 2,4- and 2,6-DNT isomers in the crude DNT is >95% by weight, based on the total weight of the crude DNT. The content of 2,4-DNT is preferably from 79.0% to 81.0% by weight, based on the sum of the weights of 2,4-DNT and 2,6-DNT. Accordingly, the content of 2,6-DNT is from 19.0% to 21.0% by weight, based on the sum of the weights of 2,4-DNT and 2,6-DNT.
The catalytic hydrogenation of these commercial DNT products can be carried out with the concomitant use of an inert solvent or without a solvent, the mixtures then being melted before the hydrogenation is carried out. It can be carried out either discontinuously or continuously using conventional reactors. In addition to a continuous reaction procedure, the selectivities of the reaction that can be achieved with the process being used, and the capacities and working lives of the catalysts used, are especially important to the economic success of the process that is used.
U.S. Pat. No. 3,356,728 discloses an improved continuous process for the preparation of aromatic amines by catalytic hydrogenation of aromatic polynitro aromatic compounds in a sludge phase reactor, in which the process is explained using the example of the reaction of dinitrotoluene. According to the teaching of U.S. Pat. No. 3,356,728, the catalytic hydrogenation of dinitrotoluene in this reaction system is carried out very effectively in terms of selectivity, catalyst working life and throughput if                the reaction zone is always saturated with hydrogen during the reaction,        the aromatic polynitro compound is added to the system while maintaining a specific weight ratio to the catalyst present in the reaction system (i.e. “catalyst loading”),        and        the concentration of the added aromatic polynitro compound in the reaction zone does not exceed a given limiting value.        
U.S. Pat. No. 3,356,728 claims a working range <0.15, and preferably a working range from 0.01 to 0.11, for the so-called catalyst loading (i.e. “ratio of the added amount of aromatic polynitro compound in kg equivalents of nitro groups per hour to the catalyst present in the reactor in kg”). It also discloses that the maximum concentration of aromatic nitro compound to be maintained in the reaction mixture is 0.1% by weight, and preferably less than 0.015% by weight, based on the weight of the reaction mixture.
According to the teaching of U.S. Pat. No. 3,356,728, the claimed catalyst loadings lead to high concentrations of active catalyst in the reaction system, such that the aromatic polynitro compound which is fed in is immediately reacted to the desired amine after entering the mixture, and the concentration of unreduced nitro compound in the reaction system is thereby kept below 0.005% by weight at all times. As disclosed in U.S. Pat. No. 3,356,728, this low concentration prevents the catalyst from rapidly being poisoned and, in addition, higher yields and improved product purity are obtained at substantially reduced costs in the reaction of the aromatic polynitro compound.
The avoidance of inadmissibly high concentrations of unreduced nitro compound in the reaction mixture of catalytic hydrogenations of aromatic polynitro compounds is also the subject-matter of U.S. Pat. No. 3,499,034. U.S. Pat. No. 3,499,034 discloses 0.5% by weight, based on the weight of the reaction mixture, as the maximum concentration of unreduced aromatic nitro compounds that is to be maintained. According to the teaching of U.S. Pat. No. 3,499,034, these low concentrations of unreduced nitro compound especially bring about low concentrations of the azoxy, azo and hydrazo compounds which, as is known, are also formed in the catalytic hydrogenation of nitro compounds and which, as described in U.S. Pat. No. 3,499,034, constitute tar-like compounds, which can be reduced but only with difficulty and only with a marked slowing down of the desired catalytic hydrogenation of the aromatic polynitro compound.
According to the teaching of EP 0 171 052 B1, the formation of tar-like intermediates in the catalytic hydrogenation of aromatic nitro compounds is dependent not only on the concentration of unreduced nitro compound but also on the nitro compound itself. As disclosed in EP 0 171 052 B1, the catalytic hydrogenation of aromatic nitro compounds is particularly successful if mixtures of at least 25% by weight of mononitro-nonamino aromatic compounds with at least 25% by weight of dinitro- or mononitro-amino aromatic compounds are used as the aromatic nitro compounds. The advantage of the disclosed reaction procedure is limited, however, in view of the outlay that it is subsequently necessary to separate the hydrogenation products by distillation. Thus, the catalytic hydrogenation of aromatic polynitro compounds on a large scale is conventionally carried out in accordance with the principles outlined by way of example in U.S. Pat. No. 3,356,728 and U.S. Pat. No. 3,499,034.
According to the teaching of GB Patent 832,153, the desired catalytic hydrogenation of the nitro compound can be greatly affected not only by the unreduced aromatic nitro compound and its intermediate azoxy, azo and hydrazo compounds, but also by contaminants contained in the nitro compound to be hydrogenated. As disclosed in GB Patent 832,153, nitrophenols and nitrocresols, which are usually present in small amounts in commercial dinitrotoluene isomeric mixtures, are decomposition accelerators as well as strong catalyst poisons, so their concentration is to be regarded as critical in respect of process safety and in respect of the efficiency of the desired catalytic hydrogenation of the nitro compound to the corresponding amine. According to the teaching of GB Patent 832,153, the nitro compound used in the catalytic hydrogenation should contain less than 500 ppm “nitrophenols”, preferably less than 20 ppm “nitrophenols”, with the term “nitrophenols” being understood according to GB Patent 832,153 as the sum of nitrophenol- and nitrocresol-like compounds.
EP 0 019 454 B1 also deals with the influence of nitrophenol-like contaminants. According to the teaching of EP 0 019 454 B1, the removal of the nitrophenol-like contaminants is largely unnecessary, but it is important in the catalytic hydrogenation of commercial dinitrotoluenes, in order to avoid catalyst poisoning and the decomposition of the amine that is formed, to lower their acid content, expressed as HNO3, to below 6000 ppm, based on the weight of the dinitrotoluene. EP 0 019 454 B1 discloses a process in which the crude dinitrotoluene is washed only with water; aqueous alkaline solutions are not used for removing nitrophenol-like contaminants.
The statements made in U.S. Pat. No. 4,482,769 are more differentiated. According to the teaching therein, washing of commercial dinitrotoluene mixtures with aqueous alkaline solutions is advantageous, but the washing should be carried out in such a manner that the aqueous phase has a pH value in the range from 5.8 to 6.4. As disclosed in U.S. Pat. No. 4,482,769, the result of such a pH value in the washing is that all the acidic components are largely removed from the dinitrotoluene, with only 2,4-dinitroorthocresol, which is poorly biodegradable, remaining in the dinitrotoluene as a secondary component. According to the teaching of U.S. Pat. No. 4,482,769, on the one hand the influence of acidic components is advantageously prevented by the claimed process, and on the other hand a low content of 2,4-dinitroorthocresol does not affect subsequent hydrogenations of the dinitrotoluene so prepared.
Surprisingly, it has now been found that in the preparation of toluenedianine, in which dinitrotoluene is reacted with hydrogen in the presence of a catalyst, the desired reaction is substantially influenced not only by the parameters known according to the prior art, but also by the carbon dioxide content of the dinitrotoluene used in the reaction. More specifically, substantially better catalyst working lives are obtained, while the selectivity of the reaction is increased, if the carbon dioxide contents of the dinitrotoluene are low.